1. Field of the Invention
The present invention relates to a multi-piece solid golf ball having a good feel upon impact, durability and an improved flight performance.
2. Prior Art
Various improvements are being made in formulating the polybutadiene used as the base rubber in golf balls so as to confer the balls with outstanding rebound characteristics.
For example, JP-A 62-89750 describes rubber compositions for use as the base rubber in solid golf balls, which compositions are arrived at by blending a polybutadiene having a Mooney viscosity of 70 to 100 and synthesized using a nickel or cobalt catalyst with another polybutadiene having a Mooney viscosity of 30 to 90 and synthesized using a lanthanide catalyst or polybutadiene having a Mooney viscosity of 20 to 50 and synthesized using a nickel or cobalt catalyst. JP-A 2-268778 describes golf balls molded using a blend composed of a polybutadiene having a Mooney viscosity of less than 50 and synthesized using a Group VIII catalyst in combination with a polybutadiene having a Mooney viscosity of less than 50 and synthesized with a lanthanide catalyst. The existing art also teaches multi-piece solid golf balls in which an intermediate layer is molded of a low-Mooney viscosity polybutadiene (JP-A 11-70187), solid golf balls molded from rubber compositions comprising a polybutadiene having a Mooney viscosity of 50 to 69 and synthesized using a nickel or cobalt catalyst in combination with a polybutadiene having a Mooney viscosity of 20 to 90 and synthesized using a lanthanide catalyst (JP-A 11-319148), solid golf balls molded from compositions based on a rubber having a 1,2 vinyl content of at most 2.0% and a weight-average molecular weight to number-average molecular weight ratio Mw/Mn of not more than 3.5 (JP-A 11-164912), golf balls molded from rubber compositions containing a high Mooney viscosity polybutadiene (JP-A 63-275356), and golf balls molded from rubber compositions comprising polybutadiene having a high number-average molecular weight in admixture with polybutadiene having a low number-average molecular weight (JP-A 3-151985). However, none of these prior-art golf balls truly satisfy all the requirements of feel upon impact, durability and flight performance.
Solid golf balls having a cover composed of inner and outer layers which have equal or substantially equal Shore D hardness are disclosed in JP-A 11-47311 and JP-A 11-47312. Although they have a satisfactory feel upon impact and durability, further improvements in flight performance are desired.
It is therefore an object of the present invention to provide multi-piece solid golf balls having a two-layer cover which are endowed with a good feel when hit with a golf club, durability and an improved flight performance.
The inventor has discovered that golf balls having a solid core and a cover of an inner cover layer and an outer cover layer, wherein the solid core is made of a rubber composition formulated from a particular type of base rubber combined in specific proportions with certain other materials, and the inner and outer cover layers have substantially equal Shore D hardness, exhibit a good synergy from optimization of the solid core materials and an appropriate distribution of hardness between the inner and outer cover layers. Multi-piece solid golf balls thus constituted have a good feel when hit with a golf club, durability and an improved flight performance.
Accordingly, the invention provides a multi-piece solid golf ball having a solid core, an inner cover layer enclosing the core, and an outer cover layer enclosing the inner cover layer. The solid core is molded from a rubber composition comprising 100 parts by weight of a base rubber composed of (a) 20 to 100 wt % of a polybutadiene having a cis-1,4 content of at least 60% and a 1,2 vinyl content of at most 2%, having a viscosity xcex7 at 25xc2x0 C. as a 5 wt % solution in toluene of up to 600 mPaxc2x7s, and satisfying the relationship: 10B+5xe2x89xa6Axe2x89xa610B+60, wherein A is the Mooney viscosity (ML1+4 (100xc2x0 C.)) of the polybutadiene and B is the ratio Mw/Mn between the weight-average molecular weight Mw and the number-average molecular weight Mn of the polybutadiene, in combination with (b) 0 to 80 wt % of a diene rubber other than component (a). The rubber composition includes also (c) 10 to 60 parts by weight of an unsaturated carboxylic acid and/or a metal salt thereof, (d) 0.1 to 5 parts by weight of an organosulfur compound, (e) 5 to 80 parts by weight of an inorganic filler, and (f) 0.1 to 5 parts by weight of an organic peroxide. The outer cover layer and the inner cover layer have a hardness difference of up to 5 Shore D hardness units.
The polybutadiene (a) is typically synthesized using a rare-earth catalyst.
Preferably, the diene rubber (b) includes 30 to 100 wt % of a second polybutadiene which has a cis-1,4 content of at least 60% and a 1,2 vinyl content of at most 5%, has a Mooney viscosity (ML1+4 (100xc2x0 C.)) of not more than 55, and satisfies the relationship xcex7xe2x89xa620Axe2x88x92550, wherein A is the Mooney viscosity (ML1+4 (100xc2x0 C.)) of the second polybutadiene and xcex7 is the viscosity, in mPaxc2x7s, of the second polybutadiene at 25xc2x0 C. as a 5 wt % solution in toluene. The second polybutadiene in component (b) is typically synthesized using a Group VIII catalyst.
In the multi-piece solid golf ball of the invention, it is generally advantageous for both the inner cover layer and the outer cover layer to have a Shore D hardness of 45 to 65.
The golf ball of the invention includes a solid core made of a rubber composition in which the base rubber is at least partly polybutadiene. It is critical that the base rubber contain as component (a) a specific amount of a polybutadiene in which the cis-1,4 and 1,2 vinyl contents, the viscosity xcex7 at 25xc2x0 C. as a 5 wt % solution in toluene, and the relationship between the Mooney viscosity and the polydispersity index Mw/Mn have each been optimized.
That is, the polybutadiene (a) has a cis-1,4 content of at least 60%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95%; and has a 1,2 vinyl content of at most 2%, preferably at most 1.7%, more preferably at most 1.5%, and most preferably at most 1.3%. Outside of the above ranges, the resilience declines.
The polybutadiene (a) must also have a viscosity xcex7 at 25xc2x0 C. as a 5 wt % solution in toluene of not more than 600 mPaxc2x7s. xe2x80x9cViscosity xcex7 at 25xc2x0 C. as a 5 wt % solution in toluenexe2x80x9d refers herein to the value in mPaxc2x7s units obtained by dissolving 2.28 g of the polybutadiene to be measured in 50 ml of toluene and carrying out measurement with a specified viscometer at 25xc2x0 C. using a standard solution for the viscometer (JIS Z8809).
The polybutadiene (a) has a viscosity xcex7 at 25xc2x0 C. as a 5 wt % solution in toluene of not more than 600 mPaxc2x7s, preferably not more than 550 mPaxc2x7s, more preferably not more than 500 mPaxc2x7s, even more preferably not more than 450 mPaxc2x7s, and most preferably not more than 400 mPaxc2x7s. Too high a viscosity xcex7 lowers the workability of the rubber composition. It is recommended that the viscosity xcex7 be at least 50 mPaxc2x7s, preferably at least 100 mPaxc2x7s, more preferably at least 150 mPaxc2x7s, and most preferably at least 200 mPaxc2x7s. Too low a viscosity xcex7 may lower the resilience.
In addition, the polybutadiene (a) must satisfy the relationship:
10B+5xe2x89xa6Axe2x89xa610B+60, 
wherein A is the Mooney viscosity (ML1+4 (100xc2x0 C.)) of the polybutadiene and B is the ratio Mw/Mn between the weight-average molecular weight Mw and the number-average molecular weight Mn of the polybutadiene. A is preferably at least 10B+7, more preferably at least 10B+8 and most preferably at least 10B+9, but preferably not more than 10B+55, more preferably not more than 10B+50, and most preferably not more than 10B+45. If A is too low, the resilience declines. On the other hand, if A is too high, the workability of the rubber composition worsens.
It is recommended that the polybutadiene (a) have a Mooney viscosity (ML1+4 (100xc2x0 C.)) of at least 20, preferably at least 30, more preferably at least 40, and most preferably at least 50, but not more than 80, preferably not more than 70, more preferably not more than 65, and most preferably not more than 60.
The term xe2x80x9cMooney viscosityxe2x80x9d used herein refers in each case to an industrial index of viscosity as measured with a Mooney viscometer, which is a type of rotary plastometer (see JIS K6300). This value is represented by the symbol ML1+4 (100xc2x0 C.), wherein xe2x80x9cMxe2x80x9d stands for Mooney viscosity, xe2x80x9cLxe2x80x9d stands for large rotor (L-type), xe2x80x9c1+4xe2x80x9d stands for a pre-heating time of 1 minute and a rotor rotation time of 4 minutes, and xe2x80x9c100Cxe2x80x9d indicates that measurement was carried out at a temperature of 100xc2x0 C.
It is desirable for the polybutadiene (a) to be synthesized using a rare-earth catalyst. A known rare-earth catalyst may be used for this purpose.
Examples of suitable catalysts include lanthanide series rare-earth compounds, organoaluminum compounds, alumoxane, halogen-bearing compounds, optionally in combination with Lewis bases.
Examples of suitable lanthanide series rare-earth compounds include halides, carboxylates, alcoholates, thioalcoholates and amides of atomic number 57 to 71 metals.
Organoaluminum compounds that may be used include those of the formula AlR1R2R3 (wherein R1, R2 and R3 are each independently a hydrogen or a hydrocarbon residue of 1 to 8 carbons).
Preferred alumoxanes include compounds of the structures shown in formulas (I) and (II) below. The alumoxane association complexes described in Fine Chemical 23, No. 9, 5 (1994), J. Am. Chem. Soc. 115, 4971 (1993), and J. Am. Chem. Soc. 117, 6465 (1995) are also acceptable. 
In the above formulas, R4 is a hydrocarbon group having 1 to 20 carbon atoms, and n is 2 or a larger integer.
Examples of halogen-bearing compounds that may be used include aluminum halides of the formula AlXnR3-n (wherein X is a halogen; R is a hydrocarbon residue of 1 to 20 carbons, such as an alkyl, aryl or aralkyl; and n is 1, 1.5, 2 or 3); strontium halides such as Me3SrCl, Me2SrCl2, MeSrHCl2 and MeSrCl3 (wherein xe2x80x9cMexe2x80x9d stands for methyl); and other metal halides such as silicon tetrachloride, tin tetrachloride and titanium tetrachloride.
The Lewis base may be used to form a complex with the lanthanide series rare-earth compound. Illustrative examples include acetylacetone and ketone alcohols.
In the practice of the invention, the use of a neodymium catalyst composed in part of a neodymium compound as the lanthanide series rare-earth compound is advantageous because it enables a polybutadiene rubber having a high cis-1,4 content and a low 1,2 vinyl content to be obtained at an excellent polymerization activity. Preferred examples of such rare-earth catalysts include those mentioned in JP-A 11-35633.
The polymerization of butadiene in the presence of a rare-earth catalyst may be carried out by bulk polymerization or vapor phase polymerization, either with or without the use of solvent, and at a polymerization temperature in a range of generally xe2x88x9230xc2x0 C. to +150xc2x0 C., and preferably 10xc2x0 C. to 100xc2x0 C.
It is also possible for the polybutadiene (a) to be obtained by polymerization using the above-described rare-earth catalyst, followed by the reaction of an end group modifier with active end groups on the polymer.
Any known end group modifier may be used. Examples include compounds of types (1) to (6) described below:
(1) halogenated organometallic compounds, halogenated metallic compounds and organometallic compounds of the general formulas R5nMxe2x80x2X4-n, Mxe2x80x2X4, Mxe2x80x2X3, R5nMxe2x80x2(xe2x80x94R6xe2x80x94COOR7)4-n or R5nMxe2x80x2(xe2x80x94R6xe2x80x94COR7)4-n (wherein R5 and R6 are each independently a hydrocarbon group of 1 to 20 carbons; R7 is a hydrocarbon group of 1 to 20 carbons which may contain a carbonyl or ester moiety as a side chain; Mxe2x80x2 is a tin atom, silicon atom, germanium atom or phosphorus atom; X is a halogen atom; and n is an integer from 0 to 3);
(2) heterocumulene compounds containing on the molecule a Y=C=Z linkage (wherein Y is a carbon atom, oxygen atom, nitrogen atom or sulfur atom; and Z is an oxygen atom, nitrogen atom or sulfur atom);
(3) three-membered heterocyclic compounds containing on the molecule the following bonds 
xe2x80x83(wherein Y is an oxygen atom, a nitrogen atom or a sulfur atom);
(4) halogenated isocyano compounds;
(5) carboxylic acids, acid halides, ester compounds, carbonate compounds or acid anhydrides of the formulas R8xe2x80x94(COOH)m, R9(COX)m, R10xe2x80x94(COOxe2x80x94R11), R12xe2x80x94OCOOxe2x80x94R13, R14xe2x80x94(COOCOxe2x80x94R15)m or the following formula 
xe2x80x83(wherein R8 to R16 are each independently a hydrocarbon group of 1 to 50 carbons; X is a halogen atom; and m is an integer from 1 to 5); and
(6) carboxylic acid metal salts of the formula R17lMxe2x80x3(OCOR18)4-l, R19lMxe2x80x3(OCOxe2x80x94R20xe2x80x94COOR21)4-l or the following formula 
xe2x80x83(wherein R17 to R23 are each independently a hydrocarbon group of 1 to 20 carbons, Mxe2x80x3 is a tin atom, a silicon atom or a germanium atom; and l is an integer from 0 to 3).
Illustrative examples of the end group modifiers of types (1) to (6) above and methods for their reaction are described in, for instance, JP-A 11-35633 and JP-A 7-268132.
In the practice of the invention, component (a) is included in the base rubber in an amount of at least 20 wt %, preferably at least 25 wt %, more preferably at least 30 wt %, and most preferably at least 35 wt %. The upper limit is 100 wt %, preferably not more than 90 wt %, more preferably not more than 80 wt %, and most preferably not more than 70 wt %.
In addition to component (a), the base rubber may include also a diene rubber (b) insofar as the objects of the invention are attainable. Specific examples of the diene rubbers (b) include polybutadiene rubber, styrenebutadiene rubber (SBR), natural rubber, polyisoprene rubber, and ethylene-propylene-diene rubber (EPDM). Any one or combination of two or more thereof may be used.
The diene rubber (b) is included together with component (a) in the base rubber in an amount of at least 0 wt %, preferably at least 10 wt %, more preferably at least 20 wt %, and most preferably at least 30 wt %, but not more than 80 wt %, preferably not more than 75 wt %, more preferably not more than 70 wt %, and most preferably not more than 65 wt %.
In the practice of the invention, it is preferable for component (b) to include a polybutadiene rubber, and especially one for which the cis-1,4 and 1,2 vinyl contents, the Mooney viscosity, and the relationship between the Mooney viscosity and xcex7 have each been optimized. The polybutadiene serving as component (b) is referred to as xe2x80x9csecond polybutadienexe2x80x9d in order to distinguish it from the polybutadiene serving as component (a).
It is recommended that the second polybutadiene in component (b) have a cis-1,4 content of at least 60%, preferably at least 80%, more preferably at least 90%, and most preferably at least 95%, and that it have a 1,2 vinyl content of at most 5%, preferably at most 4.5%, more preferably at most 4.0%, and most preferably at most 3.5%.
It is recommended that the second polybutadiene have a Mooney viscosity of at least 10, preferably at least 20, more preferably at least 25, and most preferably at least 30, but not more than 55, preferably not more than 50, and most preferably not more than 45.
In the practice of the invention, it is recommended that the second polybutadiene be one that has been synthesized using a Group VIII catalyst. Exemplary Group VIII catalysts include nickel catalysts and cobalt catalysts.
Examples of suitable nickel catalysts include single-component systems such as nickel-kieselguhr, binary systems such as Raney nickel/titanium tetrachloride, and ternary systems such as nickel compound/organometallic compound/boron trifluoride etherate. Exemplary nickel compounds include reduced nickel on a carrier, Raney nickel, nickel oxide, nickel carboxylate and organonickel complexes. Exemplary organometallic compounds include trialkylaluminum compounds such as triethylaluminum, tri-n-propylaluminum, triisobutylaluminum and tri-n-hexylaluminum; alkyllithium compounds such as n-butyllithium, sec-butyllithium, tert-butyllithium and 1,4-dilithiumbutane; and dialkylzinc compounds such as diethylzinc and dibutylzinc.
Examples of suitable cobalt catalysts include the following composed of cobalt or cobalt compounds: Raney cobalt, cobalt chloride, cobalt bromide, cobalt iodide, cobalt oxide, cobalt sulfate, cobalt carbonate, cobalt phosphate, cobalt phthalate, cobalt carbonyl, cobalt acetylacetonate, cobalt diethyldithiocarbamate, cobalt anilinium nitrite and cobalt dinitrosyl chloride. It is particularly advantageous to use the above in combination with a dialkylaluminum monochloride such as diethylaluminum monochloride or diisobutylaluminum monochloride; a trialkylaluminum such as triethylaluminum, tri-n-propylaluminum, triisobutylaluminum or tri-n-hexylaluminum; an alkyl aluminum sesquichloride such as ethylaluminum sesquichloride; or aluminum chloride.
Polymerization using the Group VIII catalysts described above, and especially a nickel or cobalt catalyst, can generally be carried out by a process in which the catalyst is continuously charged into the reactor together with the solvent and butadiene monomer, and the reaction conditions are suitably selected from a temperature range of 5 to 60xc2x0 C. and a pressure range of atmospheric pressure to 70 plus atmospheres, so as to yield a product having the above-indicated Mooney viscosity.
It is also desirable for the second polybutadiene in component (b) to satisfy the relationship:
20Axe2x88x92750xe2x89xa6xcex7xe2x80x94xe2x89xa620Axe2x88x92550, 
wherein xcex7 is the viscosity of the second polybutadiene at 25xc2x0 C. as a 5 wt % solution in toluene and A is the Mooney viscosity (ML1+4 (100xc2x0 C.)) of the second polybutadiene. The viscosity xcex7 is preferably at least 20Axe2x88x92700, more preferably at least 20Axe2x88x92680, and most preferably at least 20Axe2x88x92650, but preferably not more than 20Axe2x88x92560, more preferably not more than 20Axe2x88x92580, and most preferably not more than 20Axe2x88x92590. The use of a polybutadiene having such an optimized relationship of xcex7 and A, that suggests the high linearity of polybutadiene molecules, is effective for conferring better resilience and workability.
The second polybutadiene generally accounts for at least 30 wt %, preferably at least 50 wt %, and most preferably at least 70 wt %, and up to 100 wt %, preferably up to 90 wt %, and most preferably up to 80 wt %, of the diene rubber (b). By including the second polybutadiene within component (b) in the foregoing range, even better extrudability and hence, workability during manufacture can be conferred.
The solid core in the golf balls of the invention is molded from a rubber composition containing as essential components specific amounts of (c) an unsaturated carboxylic acid and/or metal salt thereof, (d) an organosulfur compound, (e) an inorganic filler and (f) an organic peroxide per 100 parts by weight of the base rubber.
Specific examples of unsaturated carboxylic acids that may be used as component (c) include acrylic acid, methacrylic acid, maleic acid and fumaric acid. Acrylic acid and methacrylic acid are especially preferred.
Specific examples of unsaturated carboxylic acid metal salts that may be used as component (c) include the zinc and magnesium salts of unsaturated fatty acids such as zinc methacrylate and zinc acrylate. Zinc acrylate is especially preferred.
The unsaturated carboxylic acid and/or metal salt thereof used as component (c) is included in an amount, per 100 parts by weight of the base rubber, of at least 10 parts by weight, preferably at least 15 parts by weight, and most preferably at least 20 parts by weight, but not more than 60 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much component (c) results in excessive hardness, giving the golf ball a feel upon impact that is difficult for the player to endure. On the other hand, too little component (c) undesirably lowers the resilience.
The organosulfur compound (d) of the rubber composition is essential for imparting good resilience. Exemplary organosulfur compounds include thiophenol, thionaphthol, halogenated thiophenols, and metal salts thereof. Specific examples include pentachlorothiophenol, pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, and zinc salts thereof, such as the zinc salt of pentachlorothiophenol; and organosulfur compounds having 2 to 4 sulfurs, such as diphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides, dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides. Diphenyldisulfide and the zinc salt of pentachlorothiophenol are especially preferred.
The organosulfur compound (d) is included in an amount, per 100 parts by weight of the base rubber, of at least 0.1 part by weight, preferably at least 0.2 part by weight, and most preferably at least 0.5 part by weight, but not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much organosulfur compound results in an excessively low hardness, whereas too little makes it impossible to enhance the resilience.
Examples of inorganic fillers that may be used as component (e) include zinc oxide, barium sulfate and calcium carbonate. The inorganic filler (e) is included in an amount, per 100 parts by weight of the base rubber, of at least 5 parts by weight, preferably at least 7 parts by weight, more preferably at least 10 parts by weight, and most preferably at least 13 parts by weight, but not more than 80 parts by weight, preferably not more than 50 parts by weight, more preferably not more than 45 parts by weight, and most preferably not more than 40 parts by weight. Too much or too little inorganic filler makes it impossible to achieve a golf ball core having an appropriate weight and good rebound characteristics.
The organic peroxide (f) may be a commercial product, suitable examples of which include Percumil D (manufactured by NOF Corporation), Perhexa 3M (manufactured by NOF Corporation) and Luperco 231XL (manufactured by Atochem Co.). If necessary, two or more different organic peroxides may be mixed and used together.
The organic peroxide (f) is included in an amount, per 100 parts by weight of the base rubber, of at least 0.1 part by weight, preferably at least 0.3 part by weight, more preferably at least 0.5 part by weight, and most preferably at least 0.7 part by weight, but not more than 5 parts by weight, preferably not more than 4 parts by weight, more preferably not more than 3 parts by weight, and most preferably not more than 2 parts by weight. Too much or too little organic peroxide makes it impossible to achieve a ball having a good feel upon impact and good durability and rebound characteristics.
If necessary, the rubber composition may also include an antioxidant, suitable examples of which include such commercial products as Nocrac NS-6, Nocrac NS-30 (both made by Ouchi Shinko Chemical Industry Co., Ltd.), and Yoshinox 425 (made by Yoshitomi Pharmaceutical Industries, Ltd.). The use of such an antioxidant in an amount, per 100 parts by weight of the base rubber, of at least 0 part by weight, preferably at least 0.05 part by weight, more preferably at least 0.1 part by weight, and most preferably at least 0.2 part by weight, but not more than 3 parts by weight, preferably not more than 2 parts by weight, more preferably not more than 1 part by weight, and most preferably not more than 0.5 part by weight, is desirable for achieving good rebound characteristics and durability.
The solid core of the invention can be produced by vulcanizing and curing the above-described rubber composition using a method like that employed with known rubber compositions for golf balls. For example, vulcanization may be carried out at a temperature of 100 to 200xc2x0 C. for a period of 10 to 40 minutes.
In the practice of the invention, the solid core has a hardness which is suitably adjusted according to its manner of use in the various golf ball constructions that may be employed and is not subject to any particular limitation. The core may have a cross-sectional hardness profile which is flat from the center to the surface thereof, or which varies from the center to the surface.
It is recommended that the solid core have a deflection, when subjected to a load of 980 N (100 kg), of at least 2.0 mm, preferably at least 2.5 mm, more preferably at least 2.8 mm, and most preferably at least 3.2 mm, but not more than 6.0 mm, preferably not more than 5.5 mm, more preferably not more than 5.0 mm, and most preferably not more than 4.5 mm. Too small a deformation may worsen the feel of the ball upon impact and, particularly on long shots such as with a driver in which the ball incurs a large deformation, may subject the ball to an excessive rise in spin, reducing the carry. On the other hand, if the solid core is too soft, the golf ball tends to have a dead feel when hit, an inadequate rebound that results in a poor carry, and a poor durability to cracking with repeated impact.
It is recommended that the solid core in the inventive golf ball have a diameter of at least 30.0 mm, preferably at least 32.0 mm, more preferably at least 34.0 mm, and most preferably at least 35.0 mm, but not more than 40.0 mm, preferably not more than 39.5 mm, and most preferably not more than 39.0 mm.
It is also recommended that the solid core have a specific gravity of at least 0.9, preferably at least 1.0, and most preferably at least 1.1, but not more than 1.4, preferably not more than 1.3, and most preferably not more than 1.2.
The golf ball of the invention is a multi-piece solid golf ball having a cover composed of at least two layers which are referred to herein as the xe2x80x9cinner cover layerxe2x80x9d and the xe2x80x9couter cover layer.xe2x80x9d Such cover layers can be produced from known cover stock. The cover stocks used to make both cover layers in the inventive golf ball may be composed primarily of a thermoplastic or thermoset polyurethane elastomer, polyester elastomer, ionomer resin, ionomer resin having a relatively high degree of neutralization, polyolefin elastomer or mixture thereof. Any one or mixture of two or more thereof may be used, although the use of an ionomer resin, ionomer resin having a relatively high degree of neutralization or polyester elastomer is especially preferred.
Illustrative examples of suitable commercial ionomer resins include Surlyn 6320, 8945, 9945, 8120 and 9320 (all products of E.I. du Pont de Nemours and Co., Inc.), and Himilan 1706, 1605, 1855, 1557, 1601 and AM7316 (all products of DuPont-Mitsui Polychemicals Co., Ltd.). Commercial products of polyester elastomers are Hytrel 4047, 3078, 4767 and 5557 (all products of DuPont-Toray Co., Ltd.).
Together with the primary material described above, the cover stock may include also, as an optional material, polymers (e.g., thermoplastic elastomers) other than the foregoing. Specific examples of polymers that may be included as optional constituents include polyamide elastomers, styrene block elastomers, hydrogenated polybutadienes and ethylene-vinyl acetate (EVA) copolymers.
The multi-piece solid golf ball of the invention can be manufactured by any suitable known method without particular limitation. In one preferred method, the solid core is placed within a given injection mold, following which a predetermined method is used to successively inject over the core the above-described inner and outer cover layer materials. In another preferred method, each of the cover stocks is formed into a pair of half cups, and the resulting pairs are successively placed over the solid core and compression molded.
In the golf balls of the invention, it is critical that the outer cover layer and the inner cover layer have equal or substantially equal Shore D hardness. That is, the outer cover layer and the inner cover layer should have a hardness difference of up to 5 Shore D hardness units. The hardness difference between the outer and inner cover layers should preferably be up to 4, more preferably up to 3, even more preferably up to 2, and most preferably up to 1 Shore D hardness unit.
It is recommended that both the inner and outer cover layers have a Shore D hardness of at least 45, preferably at least 48, more preferably at least 51, and most preferably at least 55, but not more than 65, preferably not more than 63, more preferably not more than 61, and most preferably not more than 60.
It is recommended that the inner and outer cover layers have a respective thickness of at least 0.7 mm, and preferably at least 1.0 mm, but not more than 3.0 mm, preferably not more than 2.5 mm, even more preferably not more than 2.0 mm, and most preferably not more than 1.6 mm.
The multi-piece solid golf ball of the invention can be manufactured for competitive use by imparting the ball with a diameter and weight which conform with the Rules of Golf; that is, a diameter of at least 42.67 mm and a weight of not more than 45.93 g. It is recommended that the diameter be no more than 44.0 mm, preferably no more than 43.5 mm, and most preferably no more than 43.0 mm; and that the weight be at least 44.5 g, preferably at least 45.0 g, more preferably at least 45.1 g, and most preferably at least 45.2 g.
Multi-piece solid golf balls according to the present invention have a good feel upon impact, durability and an improved flight performance.